The present invention relates generally to an inductor used for choke coils or other electronic parts, and more particularly to a coil-embedded dust core wherein a coil is embedded in a dust core, and its production process.
Recently achieved size reductions of electric, and electronic equipment result in the need of miniature yet high-efficiency dust cores. For the dust cores, ferrite powders and ferromagnetic metal powders are used. The ferromagnetic metal powders allow magnetic cores to decrease in size because of being higher in saturation flux density than the ferrite powders, but cause magnetic cores to have increased eddy-current looses because of their lower electrical resistance. For this reason, the surfaces of ferromagnetic metal particles in a dust core are usually provided with an insulating layer.
To achieve further size reductions of an inductor comprising a dust core, it is proposed to obtain an inductor of a structure wherein a coil is embedded in a dust core by compression molding of magnetic powders with the coil embedded therein. The inductor of this structure is herein called a coil-embedded dust core, typical examples of which are set forth in U.S. Pat. No. 2,958,807, JP-A 11-273980 and JP-B 54-28577. The coil-embedded dust cores disclosed in these publications are all produced by one single compression molding of magnetic powders and a coil charged in a molding die.
U.S. Pat. No. 3,108,931 discloses a process for producing an inductor similar to the coil-embedded dust core by compression molding of powder compacts with a coil sandwiched between them.
JP-A 3-52204 discloses a process for obtaining an inductance element similar to the coil-embedded dust core by preparing a resin ferrite core having a protrusion at its center and a resin ferrite core having a recess in its center by compression molding, coating some portions of the protrusion and a coil with an adhesive resin, applying pressure to the projection and recess fitted to the coil, and curing the adhesive resin.
Upon investigation of coil-embedded dust cores obtained by one single compression molding of a coil and magnetic powders charged in a mold as set forth in each of the first-mentioned three publications, the inventors have now found that the position of coils is prone to variations in the dust cores. Variations in the position of a coil in a dust core lead to variations in the magnetic path length and section area of the inductor, ending up with variations in the magnetic properties of the inductor. It has also been found that any deviation of the coil from its proper position in the dust core makes the coil-embedded dust core likely to crack. Any displacement of the coil from its proper position in the dust core causes local magnetic saturation, resulting in decreased inductance. Also, this may otherwise cause flux leakage from the side of the dust core nearer to the coil to become large and, hence, have some influences on an element in the vicinity of the dust core.
According to the process disclosed in U.S. Pat. No. 3,108,931, as recited in the scope of what is claimed, the first and second powder compacts are provided by pre-compression molding. Then, the first and second powder compacts are placed one over another with a coil interposed between them, and then compression molded until the interface between the first and second powder compacts is removed, thereby producing an inductor.
It is true that U.S. Pat No. 3,108,931 teaches that metal-based magnetic powders may be used; however, only ferrite powders are exemplified for the magnetic powders used therein. When powder compacts comprising metal powders are used according to the process disclosed in that patent to produce an inductor, it is more difficult to bond the first and second powder compacts together as compared with the use of powder compacts comprising ferrite powders. In other words, both powder compacts cannot be bonded together with no application of an extremely high molding pressure. Nonetheless, gaps or cracks occur between both powder compacts, and so the resultant inductor becomes poor in mechanical strength and less than satisfactory in appearance as well. On the other hand, when both powder compacts are molded at a pressure high enough to make a nearly perfect junction between them, an insulation failure occurs due to crushing of the embedded coil.
In the first example of U.S. Pat. No. 3,108,931, the second powder compact 11 is inserted into a lower molding die 10 while the first powder compact 6 molded in a cap form in an upper molding die 7 is left as such therein, as shown in FIG. 3. Then, both powder compacts are compression molded with a coil 5 interposed between them. In the second example, the first powder compact 26 of E-shape in section is molded in an upper molding die 27 and the second powder compact 34 of E-shape in section is molded in a lower molding die 30, as shown in FIG. 8. Then, both powder compacts are compression molded with a coil 5 interposed between them, while the first and second powder compacts are left as such in the upper and lower molding dies, respectively. However, the fact that the first powder compact 6 or 26 remains tightly held in the upper molding die 7 or 27 means that when the inductor is released from the mold assembly after compression molding, it is required to forcibly eject the inductor from the mold assembly by descending the upper punch. Thus, the process set forth in U.S. Pat. No. 3,108,931 does not lend itself to mass-production because of needing many releasing operations and, hence, low molding efficiency.
According to the process set forth in JP-A 3-52204 xe2x80x94which process does not rely on any compression molding of magnetic powders with a coil embedded therein, a coil is interposed between a pair of resin ferrite cores already subjected to compression molding, which are then compressed at a low pressure (of about 20 kg/cm2) and bonded together by use of an adhesive resin. For this reason, gaps are likely to occur between both cores. Now, such an inductor must be capable of being used as a surface mount device. However, the inductor disclosed in JP-A 3-52204 is of low heat resistance because the resin ferrite cores are bonded together by the resin. In other words, a problem with this inductor is that the resin ferrite cores are prone to separation from each other at a soldering step of the surface mount process.
An object of the present invention is to provide a coil-embedded dust core with a limited variation in the position of the coil located in the core. Another object of the present invention is to improve the mechanical strength of such a coil-embedded dust core. Yet another object of the present invention is to increase the productivity of such a coil-embedded dust core.
These and other objects are achieved by the embodiments of the invention as recited below.
(1) A process for producing a coil-embedded dust core by embedding a coil in magnetic powders comprising ferromagnetic metal particles coated with an insulating material, which includes:
a first compression molding step wherein one portion of magnetic powders is filled in a molding die and then compression molded to form a lower core,
a coil positioning step wherein said coil is positioned on an upper surface of said lower core in said molding die,
a coil embedding step wherein another portion of magnetic powders is again filled in said molding die in such a way that said coil is embedded in magnetic powders, and
a second compression molding step wherein pressure is applied to said lower core and said coil in a direction of lamination thereof.
(2) The coil-embedded dust core production process according to (1) above, which satisfies
1xe2x89xa6P2/P1
where P1 is a pressure applied at said first compression molding step and P2 is a pressure applied at said second compression molding step.
(3) The coil-embedded dust core production process according to (1) above, which satisfies
1 less than P2/P1
where P1 is a pressure applied at said first compression molding step and P2 is a pressure applied at said second compression molding step.
(4) The coil-embedded dust core production process according to (1) above, wherein:
said coil is a single-wound coil formed of a conductor wire of flat shape in section,
said conductor wire is wound in such a way that a major diameter direction of said flat section is perpendicular with respect to an axial direction of said coil,
said conductor wire is fixed at one end and another end with terminal electrodes, respectively, and
where said coil is positioned on the upper surface of said lower core, the terminal electrode located relatively near to said lower core is positioned on an upper surface of said conductor wire while the terminal electrode located relatively far away from said lower core is positioned on a lower surface of said conductor wire.
(5) The coil-embedded dust core production process according to (1) above, wherein the upper surface of said lower core is provided with at least one protrusion that is located on an inner and/or outer periphery of said coil.
(6) The coil-embedded dust core production process according to (5) above, wherein at least one of said protrusions, Chxe2x89xa0Dh/2, where Ch is a height of said protrusion, and Dh is a height of the coil-embedded dust core to be produced.
(7) The coil-embedded dust core production process according to (1) above, wherein Bhxe2x89xa0Dh/2, where Bh is a height of a surface of said lower core on which said coil is positioned, and Dh is a height of the coil-embedded dust core to be produced.
(8) The coil-embedded dust core production process according to (1) above, wherein the ferromagnetic powders used comprises ferromagnetic metal particles where the number of ferromagnetic metal particles having a circularity of 0.5 or less as defined by the following equation (I) accounts for 20% or less of all ferromagnetic metal particles:
circularity=4xcfx80S/L2xe2x80x83xe2x80x83(I)
where S is an area of a projected image of a particle, and L is a length of a profile of said projected image.
(9) The coil-embedded dust core production process according to (1) above, wherein the ferromagnetic metal particles used are formed of an alloy composed primarily of Fe and Ni.
(10) A coil-embedded dust core produced by the production process according to (1) above.
The inventors have now found that there is a variation in the position of a coil in a conventionally produced coil-embedded dust core for the reasons that when the coil and magnetic powders are charged in a molding die, it is difficult to hold the coil at a constant position in the molding die, and during compression molding, the coil goes down with varying amounts in the pressure application direction even when the constant pressure is applied.
In the present invention, on the other hand, only the first portion of magnetic powders is compression molded at the first compression molding step to form a lower core. Then, a coil is positioned on the upper surface of the lower core, and another portion of magnetic powders is thereafter filled for the second compression molding to form an upper core, thereby obtaining a coil-embedded dust core. By pre-forming the lower core in this way, the coil can be substantially prevented from going down during the second compression molding, and the coil can be precisely positioned prior to the second compression molding, so that the variation in the position of the core in the coil-embedded dust core can be much more reduced than ever before.
In the present invention, the lower core of the coil-embedded dust core is formed at the first compression molding step and the upper core of the coil-embedded dust core is formed at the second compression molding step. When compression molding is carried out at two stages as in the present invention, cracks may possibly occur between the lower core and the upper core due to insufficient adhesion between them. In the present invention, accordingly, there should be a relationship between the pressure P1 applied at the first compression molding step and the pressure P2 applied at the second compression molding step, such that usually 1xe2x89xa6P2/P1, and preferably 1 less than P2/P1. By limiting P2/P1 to within the preferable range, it is thus possible to substantially prevent cracks from occurring between both cores.